Microwave communication systems are widely used to provide networks for supporting communications across geographic regions. Such microwave communication systems can carry analog and digital signals. Historically, analog microwave communication systems, operated at a frequency of two gigahertz (2 GHz), have been used by railroad companies, power companies, pipeline operators and state and local governments to support private networks for voice and data communication. Digital microwave communication systems also are used to support voice and data traffic and are being used, for example, to implement digital personal communication services (PCS).
In general, microwave communication systems can include a number of microwave links arranged in a series or loop. The geographic regions between adjacent links and across which adjacent links communicate are typically referred to as microwave hops. Each microwave link typically includes a pair of radios to allow bi-directional communication across the microwave hops. Each microwave link also typically includes signal processing equipment for processing dropped lines and preparing inserted lines and for providing communication to and from local end points.
Recently, frequencies in the 1850 to 1990 megahertz (MHz) range (i.e., 1.85 to 1.99 GHz) have been allocated to providers of digital personal communication services (PCS). Various PCS providers subsequently purchased rights to use these frequencies in certain geographic areas and have established communication networks to support digital PCS. Where these new networks overlap and interfere with microwave hops of existing 2 GHz analog microwave communication systems, the PCS providers are required to relocate the operating frequencies of the existing 2 GHz microwave hops.
In this relocation effort, PCS providers often buy out the 2 GHz analog microwave communication systems and replace them with 6 or 11 GHz digital microwave systems. However, the cost of converting an entire existing analog microwave system to a digital system can be prohibitive. The conversion is costly because, in addition to the cost of converting the microwave radios and transmission equipment (such as microwave towers and antennas), there is the additional cost of converting signal processing equipment used to drop and insert lines at each microwave link. Further, there is rarely sufficient time to change an entire analog 2 GHz system to a 6 or 11 GHz digital system before PCS is scheduled to be implemented. Consequently, it is desirable for PCS providers to convert the operating frequency of only the necessary microwave hops of an existing 2 GHz analog microwave system rather than replacing the entire system with a new digital microwave system. Such a partial conversion allows the PCS provider to more quickly and inexpensively begin providing PCS in a region.
One method for converting microwave communications out of the 2 GHz operating frequency involves replacing existing 2 GHz analog radios with equivalent 6 GHz analog radios for all of the affected microwave hops. However, 6 GHz analog radios are relatively difficult and expensive to obtain and are typically available only as used equipment. For this reason, PCS providers can not rely upon 6 GHz analog radios as a solution.
Another conventional method for converting microwave communications out of the 2 GHz operating frequency involves replacing analog microwave hops with digital microwave hops at a new frequency (e.g., 6 or 11 GHz). To accomplish this conversion, the PCS provider must install necessary equipment to convert the signal to or from the analog baseband where the communication returns to analog at the end points of the digital region. The PCS provider must also install such equipment in the digital region for enabling communication to local end points. For example, in order to convert a DS3 digital signal to an analog baseband signal, the PCS provider would need to install an M13 multiplexer and twenty eight digital channel banks. With this equipment, the DS3 digital signal is processed by the M13 multiplexer to recover twenty eight DS1 signals. Each of the twenty eight DS1 signals is then fed to one of the twenty eight digital channel banks which each recover twenty four voice channels. All of the voice channels are then fed into an analog channel bank which combines the voice channels into the analog baseband. Using the same equipment, the above process can be reversed in order to convert from the analog baseband signal to the digital signal. Because the PCS provider must install such signal processing equipment in addition to replacing the analog microwave hops with digital microwave hops at the new operating frequency, this conversion method is both costly and time consuming for the PCS providers.